New research reveals how the Ice Age was extremely detrimental to the ocean’s top predator and has significantly affected diversity among today’s living populations.

Being at the top of the food chain, killer whales are considered to be sentinel species – organisms that can act as an early warning for environmental change.

A new study, published in the journal Molecular Biology and Evolution, sought to investigate past demographic patterns of the killer whale, and identify the main factors determining population fluctuation over long periods of time. The results reveal patterns of past climate change that may have impacted on food availability and also the effect of the last Ice Age on the diversity of the killer whale.

Researchers, including Dr Andre Moura and Dr Malgorzata Pilot, from the School of Life Sciences, University of Lincoln, UK, working with killer whale expert Professor Rus Hoelzel from Durham University, UK, concluded that killer whales experienced a rapid decline and evolutionary bottleneck during the last Ice Age (110,000-12,000 years ago).

First author Dr Moura said: “In the ocean, the killer whale rules as a top predator, feeding on everything from seals and sharks to the great whales. Being at the top of the food chain they are sensitive to disturbances in prey resources, and can therefore teach us many things about past and future ocean ecosystems and environmental change.

“Our results indicate that events concurrent with the last glacial period induced a severe population decline in this top marine predator, possibly related to drastic changes in the oceans' productivity.”

Although whole genome data from northern hemisphere animals indicates that most populations declined, 616 individual mitochondrial DNA samples suggest at least one population retained diversity in a stable, productive ecosystem off southern Africa.

The research team concluded that environmental changes during the last glacial period contributed to the pattern of diversity among surviving populations, and that the relatively high diversity of the South African population suggests that changes in ocean productivity could be a factor associated with widespread decline.

Dr Moura added: “This was the first time the South African population was analysed, and its level of diversity surprised everyone. The killer whale has a notorious lack of diversity elsewhere, which is confirmed by the genomes we analysed. This discrepancy suggests that the South African population might have been a glacial refuge for this species. However, the convergence of whales from diverse populations into the region in the recent past is also a possible explanation.

“The association between this disproportionally diverse population and a productive environment that has remained stable for quite a long time suggests that ocean productivity may have been a key factor and favours the refuge hypothesis.”

The decline of the killer whale suggests substantial changes in marine ecosystems correlated with climate change, and the possibility of a similar impact on other marine top predators.

Alternative explanations for population decline include disease epidemics and exploitation, and it is likely that the factors affecting killer whale population dynamics and distribution are overlapping and complex.